A single-phase power factor correction (PFC) circuit is widely used in switching power supplies energized using commercially supplied power, such as in telecommunication power supplies, uninterrupted power supplies (UPS), and the like. FIG. 1 depicts an exemplary, known power factor correction circuit for enabling input current to meet the International Engineering Consortium (IEC), requirements relevant to a harmonic current standard and for setting the power factor approximately to unity.
A single-phase power factor correction circuit generally utilizes a boost converted or boost circuit. A traditional boost converter operates as follows:    1. The duty cycle of power switch S1 is controlled by controlling on and off periods of power switch S1, to provide a boost function and a voltage regulation function for the output voltage. The output voltage may be described with the following equation: VO=VIN/d, wherein d is the duty cycle of the power switch S1.    2. When the power switch S1 is on, an input voltage is applied across the two ends of an inductor Lm to charge Lm and store energy therein. Thus, the current through Lm rises, and freewheeling diode D is reverse biased and turned off.    3. When the power switch S1 is off, freewheeling diode D turns on because the induced current through Lm cannot change abruptly. The input voltage is connected to inductor Lm in series and a current passing through freewheeling diode D supplies power to the output capacitor Co and a load (not shown), reducing the induced current through Lm.
The boost circuit of FIG. 1 has certain features which can be less than desirable. For example, when power switch S1 is turned off, freewheeling diode D turns on, and a positive current flows through it. When the power switch S1 is on, a negative voltage is provided across freewheeling diode D. Freewheeling diode D cannot then be turned off immediately because of the recovery effect of the freewheeling diode D. Current can then flow through reverse biased freewheeling diode D, forming a reverse recovery current. The reverse recovery current and the current through the inductor Lm flow through power switch S1, thereby increasing loss when power switch S1 turns on, and also increasing the loss of freewheeling diode D. When the output voltage rises, the above-described effect increases because the time of reverse recovery of the diode increases. The higher the switching frequency of power switch S1, the greater the loss caused by the reverse recovery current. The above-described effect thus limits the operating frequency of the circuit.
To address the above-described limitations, an auxiliary inductor is connected in series with freewheeling diode D to reduce the reverse recovery current. The auxiliary inductor, however, can only be used to reduce the reverse recovery current, but does not reduce the energy associated with the reverse recovery current. An additional capacitor may be required to store the reverse recovery energy until the freewheeling diode D turns on again and supplies the output capacitor Co with the stored energy. The addition of a storage capacitor provides a second circuit branch for the current of inductor Lm circuit. When the current through inductor Lm is relatively high, all current flowing in the second branch circuit may not be transferred to the branch circuit of the auxiliary inductor when power switch S1 turns off because the reverse recovery energy in the storage capacitor may not be sufficient. Therefore, auxiliary inductor Ls cannot completely reduce the reverse recovery current.